In process technology as well as in automation technology, field devices are often applied, which serve for registering and/or influencing process variables. Serving for registering process variables are measuring devices, such as, for example, fill level measuring devices, flow measuring devices, pressure- and temperature measuring devices, pH measuring devices, conductivity measuring devices, etc., which register the corresponding process variables, fill level, flow, pressure, temperature, pH-value, and conductivity, respectively. Used for influencing process variables are actuators, such as valves or pumps, via which e.g. the flow of a liquid in a pipeline or the fill level of a medium in a container is changed. Referred to as field devices are, in principle, all devices, which are applied near to the process and which deliver, or process, process relevant information. A large number of such field devices are available from the group of firms, Endress+Hauser. In connection with the invention, the concept, field device, thus, refers to all types of measuring devices and actuators. Furthermore, the concept, field device, includes, however, also e.g. a gateway, a radio adapter or other bus participants integrated/integrable in a bus system.
Control occurs, in such case, from a control system, which is connected with the individual field devices via a data bus. All information needed for the control are exchanged between the control system and the field devices via the data bus, also called a fieldbus.
An often applied data-, or fieldbus, works according to the HART standard of the HART® Foundation. Field devices working according to the HART standard are also referred to as HART devices.
Besides the transmission of measured values, field devices allow also the transmission of other kinds of information stored in the field device, information such as e.g. configuration- and/or parameter data (zero-point, measured value range, etc.), measurement curves as well as diagnostic information. The parameters can be set by the control system or from a service- and display device.
This procedure is also referred to as configuring and/or parametering of the field device. The data for this are referred to as configuration- and/or parametering data.
In the case of each initial installation or in the case of a device replacement, the field device must be configured and/or parametered.
For this, a service- and/or observation software runs, most often, on a portable computer unit (PC, laptop), which is connected, for example, via a USB- or also a serial, COM interface (e.g. RS232, RS485) with an apparatus connected to the fieldbus, a so-called HART communication box, for transmission of the configuration- and/or parametering data.
In order to establish a HART-standard conforming communication between such a HART communication box and a field device, an impedance matching is required, since only such a so-called “low-impedance” (acronym: LI) installation is permitted within the corresponding electrical current loop.
Since current field devices have both “low-impedance” as well as also “high-impedance” (acronym HI) interfaces, a service technician must take care that the HART communication box is appropriately matched and connected to the electrical current loop. This can occur, for example, using a particular HART communication box, in the case of which a corresponding communication resistor (also called the load) is switchable in by means of a switch. Alternatively, a service technician can manually introduce a corresponding resistor into the existing installation.
Both variants assume that the service technician knows whether a communication resistor must be connected or whether a communication resistor is already present and, thus, does not have to be switched in.
In the case of both variants, for example, by incorrect positioning of the manual switch of the HART communication box or also by manual introduction of an additional communication resistor not actually required, problems can arise as follows:                1.) If in the installation, for instance in the electrical current loop, two communication resistors are connected in parallel when they should not be, the impedance and therewith the signals, i.e. the signal amplitudes, can lessen in such a manner that communication is no longer possible.        2.) If in the installation no communication resistor is present and two “high-impedance” devices are connected together, the signals, i.e. the signal amplitudes, get larger and the inputs of the communication box can be overloaded. This can lead to communication problems or even to complete loss of communication.        
An object of the invention is to enable an as defect free as possible HART communication connection.